The addition of temozolomide (TMZ) during and after radiation therapy (RT) improved survival for patients with newly diagnosed GBM and is the current standard of care. However, the survival benefit of TMZ therapy is limited by the development of TMZ resistance in almost all patients, and there is significant interest in identifying molecular sensitizing strategies to improve the efficacy of TMZ. One promising strategy targeting the repair of TMZ-induced DNA damage is inhibition of poly-ADP-ribose polymerase (PARP). PARP is indirectly involved in numerous repair pathways, and previous data suggest that PARP inhibitors will sensitize essentially all tumors to TMZ. While our preliminary in vivo data in a panel of primary GBM xenografts confirms a robust sensitizing effect of the PARP inhibitor ABT-888 when combined with TMZ or TMZ/RT, our data also demonstrate that combination therapy with PARP inhibitors is only effective in tumors that are inherently sensitive to TMZ. Defects in homologous recombination (HR)- mediated DNA repair are associated with increased sensitivity to TMZ and to PARP inhibitor therapy. In Aim 1, the relationship between the TMZ-sensitizing effects of PARP inhibitors and molecular defects in key HR repair genes will be tested in primary GBM xenograft models. Preliminary data presented suggest that radiation can affect the emergence of TMZ resistance and that TMZ resistance can adversely impact on the efficacy of PARP inhibition. In Aim 2, the relationship between PARP inhibition, TMZ resistance and radiation responses will be explored to provide additional insight into potential mechanisms of resistance to the TMZ-sensitizing effects of PARP inhibitors. Finally, TMZ sensitivity is critically controlled by silencing of the MGMT repair protein by promoter methylation, and in Aim 3 MGMT methylation, and potentially other molecular features defined in Aim 1, will be used to select patients for enrollment on a clinical trial evaluating the efficacy of ABT-888 combined with chemo-radiotherapy. Collectively, these studies may provide a rational basis for customized molecular therapy with PARP inhibitors combined with chemo-radiotherapy in patients with newly diagnosed GBM.